The present invention relates to apparatus for the dynamic testing of materials and structures and more particularly to a load cell for use in such apparatus.
Such apparatus is well known and FIG. 1 shows a typical arrangement where a specimen 10 to be tested is mounted in a framework 11 by means of two grips 12 and 13. One of the grips, in this case the grip 13, is connected to a suitable load applying device such as an hydraulic actuator for moving the grip while the other grip, in this case the grip 12, is connected to a load cell for monitoring the forces placed on the specimen by the load applying device. When testing dynamically, utilising frequencies over say 10 Hz, the force indicated by the load cell 15 equals the force in the specimen 10 and the inertia force generated by the grip between it and the load cell, in this case the grip 12. In other words, this can be expressed as:
Fcell=Fspecimen+ma
Where Fcell is the force seen at the load cell, Fspecimen is the force seen at the specimen, xe2x80x9cmxe2x80x9d is the mass of the grip or fixture between the specimen and the cell and xe2x80x9caxe2x80x9d is the acceleration of the grip or fixture. The inertia force xe2x80x9cmaxe2x80x9d is of course an unwanted error signal and needs to be removed in order to measure specimen forces accurately.
Historically, the error caused by the inertia force has either been unappreciated or else ignored. However, the error magnitude can easily amount to 10% of the actual force and considerably more if mechanical resonances in the system enhance the local grip acceleration. As a result, fatigue data accuracy is potentially compromised. This can have serious safety consequences when the parts undergoing fatigue testing in service are aircraft or automotive components such as critical fasteners.
Further on uncompensated systems, inertia force errors at high frequencies, circa 100 Hz, can cause load control loops to become prematurely unstable, particularly when testing compliant specimens.
Previously it has been proposed to try and correct the inertia force error by using a separate accelerometer fixed to the grip between the specimen and the cell in order to provide a proportional signal which is used to correct the load cell output signal. While this system was an improvement on the original uncompensated system, it still did not totally correct for the inertia force error.
It is an object of the invention to provide accurate correction of the error caused by the grip inertia in an apparatus for dynamically testing objects.
In the present invention, this is achieved by placing an accelerometer within the load cell on the same axis as the force applied to the object under test.
Preferably, the signal from the accelerometer is fed with a signal derived from the load cell down a single cable and then combined with the load cell signal at the testing machine structural rig controller and digitised.
In addition to providing an accurate result, dynamic calibration of the apparatus can be automated enabling rapid optimisation of the apparatus to a new set-up where the grip mass may have changed. Also, the frequency range over which valid testing can be carried out is increased, enabling fatigue tests to be completed much more quickly. As an example, the time to apply ten million cycles (typical for high cycle fatigue testing) at 20 Hz is about 6 days non-stop whereas at 60 Hz it reduces to 2 days. This offers major cost savings to users of existing fatigue testing machines by upgrading their load cells and controller.
Furthermore, inertia compensation enables a much tighter load control loop giving greater bandwidth and more accurate control.
Finally, the invention provides good force measurement and control when the load cell is mounted to the moving end of the actuator piston, where accelerations are significant, such as in many bio-mechanics and structural rig applications.